1. Field of the Invention
The present invention relates to a calcium fluoride crystal and an optical system and an apparatus using the same. In particular, the present invention relates to a calcium fluoride crystal used for an optical article which transmits excimer laser light used in exposure apparatuses for photo-lithography.
2. Description of the Related Art
Excimer lasers have attracted attention as the only high-output lasers which can oscillate in an ultraviolet region, and have been applied in electronic, chemical and energy industrial fields. In these industrial fields, the excimer laser is used for processing metal, resin, glass, ceramic and semiconductor articles as well as for chemical reactions.
The equipment for generating an excimer laser beam is known as an excimer laser oscillator. A laser gas such as Ar, Kr, Xe, F2, or Cl2 which is filled into a chamber is excited by electron beam irradiation or electric discharge. The excited atoms bond to atoms in a ground state to form molecules which can be present in an excited state. Such molecules are called excimers. The excimers are unstable and immediately return to the ground state, simultaneously emitting ultraviolet light. This phenomenon is referred to as bond-free transition. An excimer laser oscillator amplifies ultraviolet light formed by the bond-free transition with an optical resonator comprising a pair of mirrors and outputs it as a laser light beam.
Among excimer laser beams, the KrF laser beam (248 nm) and the ArF laser beam (193 nm) operate in a vacuum ultraviolet region, respectively. Optical articles must have high transparencies in such a region. Fluorite (calcium fluoride single-crystal) is suitable for such optical articles.
Although conventional fluorite exhibits satisfactory characteristics in general visible light optical articles, its optical characteristics deteriorate during repeated irradiation cycles of high output light having a short wavelength, such as in an excimer laser beam.
The present inventors have found that the deterioration is induced by the crystal structure and impurities.
It is an object of the present invention to provide a calcium fluoride crystal in which light transmittance characteristics do not deteriorate by consecutive irradiation of high output shortwave light over long time periods.
It is another object of the present invention to provide a calcium fluoride crystal preferably used in an excimer laser optical article, particularly, for an exposure apparatus for photo-lithography.
It is a further object of the present invention to provide a calcium fluoride crystal which can produce a highly reliable optical article.
It is still another object of the present invention to provide a calcium fluoride crystal which can be produced with low production costs.
It is still a further object of the present invention to provide an optical part for excimer lasers having a large diameter of 250 mm or more, of which optical characteristics do not deteriorate during repeated irradiation cycles of high output light having a short wavelength.
It is yet another object of the present invention to provide an exposure apparatus for photo-lithography which is capable of stably exposing fine patterns of 0.25 microns or less for long periods.
In accordance with a first aspect of the present invention, a calcium fluoride crystal has an internal transmittance of 70% or more for light of 135-nm wavelength or more.
In accordance with a second aspect of the present invention, a calcium fluoride crystal contains strontium, and the strontium content ranges from 1 ppm to 600 ppm.
The strontium content preferably ranges from 20 ppm to 400 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a third aspect of the present invention, an optical system for excimer lasers comprises a lens of a calcium fluoride crystal having an internal transmittance of 70% or more for light of a 135-nm wavelength or more.
In accordance with a fourth aspect of the present invention, an optical system for excimer lasers comprises a lens of a calcium fluoride crystal containing strontium, and the strontium content ranges from 1 ppm to 600 ppm.
In accordance with a fifth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal having an internal transmittance of 70% or more for light of a 135-nm wavelength or more and a stage for holding a substrate being exposed.
In accordance with a sixth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal containing strontium and a stage for holding a substrate being exposed, the strontium content ranging from 1 ppm to 600 ppm.
In accordance with a seventh aspect of the present invention, a calcium fluoride crystal contains aluminum, and the aluminum content is 50 ppm or less.
The aluminum content preferably ranges from 1 ppm to 50 ppm. The aluminum content more preferably ranges from 1 ppm to 5 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with an eighth aspect of the present invention, an optical system for excimer lasers comprises a calcium fluoride crystal containing aluminum, the aluminum content being 50 ppm or less.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a ninth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal containing aluminum and a stage for holding a substrate being exposed, the aluminum content being 50 ppm or less.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a tenth aspect of the present invention, a calcium fluoride crystal contains silicon, and the silicon content is 50 ppm or less.
The silicon content preferably ranges from 1 ppm to 50 ppm. The silicon content more preferably ranges from 1 ppm to 5 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with an eleventh aspect of the present invention, an optical system for excimer lasers comprises a calcium fluoride crystal containing silicon, the silicon content being 50 ppm or less.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a twelfth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal containing silicon and a stage for holding a substrate being exposed, the silicon content being 50 ppm or less.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-in wavelength or more.
In accordance with a thirteenth aspect of the present invention, a calcium fluoride crystal contains magnesium, and the magnesium contentranges from 1 ppm to 10 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-mm wavelength or more.
In accordance with a fourteenth aspect of the present invention, an optical system for excimer lasers comprises a calcium fluoride crystal containing magnesium, the magnesium content ranging from 1 ppm to 10 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a fifteenth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal containing magnesium and a stage for holding a substrate being exposed, the magnesium content ranging from 1 ppm to 10 ppm.
The internal transmittance of the calcium fluoride crystal is preferably 70% or more for light of a 135-nm wavelength or more.
In accordance with a sixteenth aspect of the present invention, a calcium fluoride crystal has an internal transmittance of 70% or more for light of a 135-nm wavelength or more and contains lanthanum (La) and yttrium (Y), the lanthanum content being 1 ppm or less and the yttrium content being 10 ppm or less.
The calcium fluoride crystal may further contain at least one element selected from the group consisting of strontium (Sr), aluminum (Al), silicon (Si) and magnesium (Mg).
In accordance with a seventeenth aspect of the present invention, an optical part for excimer lasers comprises a calcium fluoride crystal having an internal transmittance of 70% or more for light of a 135-nm wavelength or more, and containing lanthanum (La) and yttrium (Y), the lanthanum content being 1 ppm or less and the yttrium content being 10 ppm or less.
In accordance with an eighteenth aspect of the present invention, an exposure apparatus for photo-lithography comprises an optical system comprising a lens of a calcium fluoride crystal having an internal transmittance of 70% or more for light of a 135-nm wavelength or more and containing lanthanum (La) and yttrium (Y), and a stage for holding a substrate being exposed, the lanthanum content being 5 ppm or less and the yttrium content being 10 ppm or less.
In accordance with a nineteenth aspect of the present invention, an exposure apparatus comprises an illuminating light source, a stage for placing an exposed member and a plurality of optical articles provided in a light optical system and/or a projection optical system, at least one of the optical article comprising a calcium fluoride crystal having an internal transmittance of 70% or more for light of a 135-nm wavelength or more.
The illuminating light source may be an excimer laser light source.
In accordance with a twentieth aspect of the present invention, a method of making a calcium fluoride crystal is characterized in that the calcium fluoride crystal has a transmittance of 70% or more for light of a 135-nm wavelength or more.
In this method, a step for mixing, melting and cooling calcium fluoride and a scavenger may be repeated at least twice.
The scavenger may be zinc fluoride, bismuth fluoride, sodium fluoride, or lithium fluoride.
The calcium fluoride crystal in accordance with the present invention can be produced at relatively low cost and used as a highly reliable optical article. Deterioration in the optical characteristics of the optical part for excimer lasers using the calcium fluoride crystal does not occur during repeated irradiation of shortwave, high-output light for long time periods. The exposure apparatus for photo-lithography can stably expose fine patterns of 0.25 microns or less for long time periods.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.